1. Field
The present invention is a fast acting digital simulation model of sampled data in a two-dimensional input scene to simulate forward looking infrared (FLIR) sensors wherein the optical modulation transfer function of the FLIR is determined in the spatial domain rather than the frequency domain.
2. Prior Art
In the part, the FLIR systems have been modeled, i.e. simulated, with a mathematical equation for the FLIR system performance analysis. The most popular model is the U.S. Army night vision laboratory static performance model for a thermal viewing system, herein called the night vision model. However, this model is for a one dimensional analog FLIR system only. However, a two-dimensional digital simulation has a more stringent requirement than by one-dimensional analog. For example, sampling in these digital FLIR systems causes aliasing problems when the optics are after the down sampled two dimensional input. A new trend has developed to use image processors as a means for modeling the two dimensional digital FLIR system wherein an operator may use a workstation, i.e. a keyboard and a cathode ray tube (CRT), of the image processor to provide not only numerical values but pictorial representations of a simulated FLIR sensor.
The input image data from a two-dimensional simulation is usually very large, typically 1024.times.1024 pixels or higher. The prior art simulation used a fast Fourier transform (FFT) operation on this large amount of input image data to transform the data from the two dimensional spatial domain to the frequency spectrum domain which is usually used for determining the optics modulation transfer function (MTF) and then applying an inverse FFT to convert the image data from the frequency domain back to the spatial domain. These FFT and FFT processes require an intense calculation process which consumes most of the central processor units (CPU's) time. The time required is typically about 45 minutes for a 512.times.512 pixel image display, an excessive amount of time for an operator to wait for an observation of the optics MTF. The present invention discloses a simulation of a FLIR sensor which does not use the time consuming FFT and inverse FFT operations but instead determines the optics MTF in the spatial domain and has an output therefrom in the spatial domain. The optics are also positioned prior to the detector downsampling to avoid aliasing of the input scene signal since the optics have a finite bandwidth which is a much wider bandwidth than the detectors. The optics prefilter the frequency spectrum of the input scene to remove the low amplitude high frequency curves therefrom prior to down sampling. In the prior art simulation, the downsampled frequency spectrum curves overlapped causing the aliasing and thus deterioration of the input scene which highly limited a true assessment of the simulation.